A wavelength slicer uses a waveguide grating router having a periodic arrangement of reflectors in the output circular boundary of the router, which causes each input signal to pass twice through the router. The slicer includes two interleaved sets of equally spaced reflectors. Each set is slightly displaced from the output curve, and their displacements are properly chosen so as to effectively produce a phase shift of π/2 between the reflected signals. The reflected signals again pass through the router to produce two separate signals, containing respectively the even and odd channels of the input signal. A second slicer embodiment is realized using two gratings that are coupled to elliptical and circular reflectors. A wavelength filter is realized by forming on the output curve a set of equally spaced reflectors with spacing equal to an integer fraction of the spacing of the images.
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12. An optical apparatus comprising
a first free space region having an input boundary, an output boundary, and a first input waveguide connected to the input boundary for receiving an input signal, a second free space region having an input boundary and an output boundary, a waveguide grating including a plurality of radiating arms each arm having a first end connected to the output boundary region of the first free space region and a second end connected to input boundary of the second free space region, and wherein the second free space region includes a periodic segmented reflector formed on its output circular boundary region, the segmented reflector having period essentially equal to an integer fraction of the spacing of the various image orders, so that the various image orders of each wavelength interval of the input signal are equally reflected, each from its respective segment. 1. An optical imaging apparatus comprising a grating having an input curve and an output image curve produced with specified magnification M, the optical imaging apparatus further comprising
an input waveguide connected to a location P of the input curve for receiving an input signal at a particular input wavelength, the input signal being dispersed by the various orders of the grating into a set of equally spaced images produced on the output image curve with spacing Ω, the dispersion by the grating causing the locations of the output images on the output image curve to vary with wavelength and a periodic arrangement A of reflective elements located along the output image curve with period essentially equal to an integer fraction of the spacing Ω of the output images, where the integer can be equal to unity, each reflective element essentially reversing the propagation direction of an image intercepted by that element, the periodic arrangement thereby causing the set of equally spaced images to produce reflected signals which pass back through the grating to the input waveguide, the reflected signals are efficiently transferred back to the input waveguide, thereby causing an output signal traveling in the input waveguide in a direction opposite that of the input signal.
23. A wavelength slicer comprising
a first router including a first free space region, a second free space region, a waveguide grating connected between said first and second free space regions, and an input waveguide connected to the input side of the first free space region for inputting an input signal having multiple equally spaced wavelengths; a second router including a first free space region, a second free space region, a waveguide grating connected between said first and second free space regions of the second router, an output waveguide connected to an input side of the first free space region of the second router, and wherein said second free space region of said second router overlaps said second free space region of said first router, wherein the shared second free space region includes an elliptical reflector and a circular reflector, and wherein input signal images are radiated from the waveguide grating of the first router so as to impinge on the reflector arrangement, the circular reflector being located so as to reflect the odd numbered wavelengths of the radiated input signal images back through the waveguide grating of the first router to the input waveguide and the elliptical reflector being located so as to reflect the even numbered wavelengths of the input signal through the waveguide grating of the second router to the output waveguide.
2. The optical apparatus of
a circulator device connected between the input signal and the input waveguide so that its first port connects to the input signal, its second port connects to input waveguide, and its third port outputs a signal exiting the input waveguide.
3. The optical apparatus of
a second input waveguide connected to a location Q of the input curve, the two input waveguides forming two separate input ports with locations P and Q separated by a specified lateral displacement d, the periodic arrangement of reflective elements A is interleaved with a second periodic arrangement of reflective elements B, so that each element A is adjacent to element B, and the spacing of the two elements is approximately equal to the image Md of the input displacement d, the two elements A and B being characterized by reflections of similar magnitudes but different phases that approximately differ by π/2.
4. The optical apparatus of
a circulator device connected between the second input signal and the second input waveguide so that its first port connects to the second input signal, its second port connects to second input waveguide, and its third port outputs a signal exiting the second input waveguide.
5. The optical apparatus of
a 2×2 coupler having two input ports and two output ports, each output port connected to a different one of the first and second input waveguides, the optical apparatus thereby forming a reflective arrangement which is characterized by four reflection coefficients, each reflection coefficient being produced at a particular input port of the 2×2 coupler by an input signal applied to one of the two input ports, the reflective arrangement, in response to a multiple wavelength input signal applied to the first input port, producing a reflected signal back to the first input port which has a wavelength response characterized by two sets of interleaved wavelengths U and V, with one set U representing passbands of efficient reflection back to the first input port, and the other set V representing stopbands of substantially lower reflection back to the first input port.
6. The optical apparatus of
a circulator device connected between the input signal and the 2×2 coupler so that its first port connects to the input signal, its second port connects to a first input port of the 2×2 coupler, and its third port outputs a signal exiting the first input port of the 2×2 coupler.
7. The optical apparatus of
8. The optical apparatus of
9. The optical apparatus of
the optical apparatus thereby forming a reflective imaging arrangement with two input waveguides characterized by four reflection coefficients, each reflection coefficient produced in a particular input waveguide by a signal applied to one of the two input waveguides, such that the wavelength response produced by each of the four reflection coefficients is characterized by two sets of interleaved wavelength intervals, with one set representing passbands of efficient reflection, and the other set representing stopbands of substantially lower reflection.
11. The optical apparatus of
13. The optical apparatus of
14. The optical apparatus of
the integer is 2 and alternate segments have different reflection characteristics.
15. The optical apparatus of
16. The optical apparatus of
the segmented reflector includes an arrangement of two periodic interleaved sets of reflector segments, A and B, the two interleaved sets imparting a π/2 difference in phase shift to signals reflected therefrom, the first free space region includes a second input waveguide connected to the input boundary for outputting a reflected signal, and wherein the connections of the first and second waveguides to the input boundary, P and Q, are separated by a specified lateral displacement d, the spacing between adjacent segments A and B being approximately equal to the image Md of the input displacement d, the two segments A and B being characterized by reflections of similar magnitudes.
17. The optical apparatus of
18. The optical apparatus of
a 2×2 coupler having two input ports and two output ports, each output port connected to a different one of the first and second input waveguides, the optical apparatus thereby forming a reflective arrangement which is characterized by four reflection coefficients, each reflection coefficient is produced at a first input port of the 2×2 coupler by an input signal applied to that port or to a second input port of the 2×2 coupler, the reflective arrangement, in response to a multiple wavelength input signal applied to the first input port, producing a reflected signal back to the first input port which has a wavelength response characterized by two sets of interleaved wavelengths, with one set representing passbands of efficient reflection back to the first input port, and the other set representing stopbands of substantially lower reflection back to the first input port.
19. The optical apparatus of
when an input signal, including a sequence of wavelengths λ1, λ2, . . . λn, where λ1<λ2<. . . <λn, is applied to a first input port of the 2×2 coupler, each component signal outputted at each of the two output ports of the 2×2 coupler forms a plurality of images, where each image impinges a predetermined segment of the segmented reflector depending on the particular wavelength present in said each image, wherein the adjacent segments of the segmented reflector introduce a π/2 difference in phase shift to the reflected image, and wherein the even numbered wavelengths of said sequence of wavelengths said plurality of wavelengths are outputted at the first port of the 2×2 device and the odd numbered wavelengths of said the sequence of wavelengths are outputted at a second port of the 2×2 device.
20. The optical apparatus of
a circulator device connected between the input signal and the 2×2 coupler so that its first port connects to the input signal and its second port connects to a first input port of the 2×2 coupler and its third port outputs a signal exiting the first input port of the 2×2 coupler.
21. The optical apparatus of
a second circulator device :having a first port for receiving a second input signal including a plurality of wavelengths, a second port for coupling the second input signal into the second input port of the 2×2 coupler, and a third port for outputting a signal exiting the second input port of the 2×2 coupler.
22. The optical apparatus of
Ω is the spacing between images orders and is approximately
and where R is the distance of a radiating arm from the arms focal point, λ is the wavelength and a is the period of the grating.
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This invention relates to wavelength-division multiplexing in optical systems, and more particularly to apparatus for combining and/or separating combs of equally spaced wavelength channels.
An important function that must be provided in high quality optical networks is that of wavelength multiplexing and demultiplexing a plurality of signals of different wavelengths. In particular, an important device for performing this function is the interleaver (also referred to herein as a slicer) which separates an input signal consisting of closely spaced wavelength signals, into two sets of equally spaced wavelength channels, each set having twice the spacing of the interleaved signal. The interleaver must have low loss, preferably less than 3 dB, and it must be approximately characterized by rectangular transfer functions.
A technique that is often used for combining and separating signals of various channels in wavelength-division multiplexing is the wavelength router. A rectangular transfer function can be realized by concatenating two such routers, but such an arrangement is difficult to realize on a single wafer using conventional techniques for two reasons. The first reason is that the loss of a conventional router typically exceeds 3 dB, which would result in two concatenated routers having a total loss of more than 6 dB. Another reason is that the two-router arrangement is difficult to realize on a planar wafer because of the size of the two routers, particularly when the channel spacing is equal to the free-spectral range of the routers, in which case one finds that the layout of the two routers typically overlap.
Thus there is a continuing need for a low loss interleaver or slicer having rectangular wavelength transfer functions.
In one embodiment of my new invention, a low loss wavelength slicer is implemented using a waveguide grating router connected to an input 2×2 coupler. The router is terminated by an output reflective arrangement causing each reflected signal to pass twice through the router. The reflective arrangement consists of two interleaved sets of reflective elements located in the immediate vicinity of the output image curve of the router. Each set is slightly displaced from the output curve, and their displacements are properly chosen so as to effectively produce a phase shift of π/2 between the signals reflected by the two sets. The reflected signals then pass again through the router and the 2×2 coupler which produces two separate output signals, containing respectively the even and odd channels of the input signal, which are each outputted from separate input ports of the 2×2 coupler. In another embodiment, a wavelength filter is realized by forming on the output curve a set of equally spaced reflectors with spacing equal to an integer fraction of the spacing of the images. A second slicer embodiment is realized by using two routers or gratings that are coupled to elliptical and circular reflectors that separate the even and odd channels.
More generally my invention is directed to an optical imaging apparatus comprising a grating having an input curve and an output image curve produced with specified magnification M, the optical imaging apparatus further comprising
(1) an input waveguide connected to a location P of the input curve for receiving an input signal at a particular input wavelength, the input signal being dispersed by the various orders of the grating into a set of equally spaced images produced on the output image curve with spacing Ω, the dispersion by the grating causing the locations of the output images on the output image curve to vary with wavelength and
(2) a periodic arrangement A of reflective elements located along the output image curve with period essentially equal to an integer fraction of the spacing Ω of the output images, where the integer can be equal to unity, each reflective element essentially reversing (reflecting over the same path) the propagation direction of an image intercepted by that element, the periodic arrangement thereby causing the set of equally spaced images to produce reflected signals which pass back through the grating to the input waveguide, the power reflected signals are efficiently transferred back to the input waveguide, thereby causing an output signal traveling in the input waveguide in a direction opposite that of the input signal.
In a second arrangement, a second input waveguide is connected to a location Q of the input curve, so as to realize two separate input ports with locations P and Q separated by a specified lateral displacement d, the periodic arrangement of reflective elements A is interleaved with a second periodic arrangement of reflective elements B, so that each element A is adjacent to an element B, and the spacing of the two elements is approximately equal to the image Md of the input displacement d, the two elements A and B being characterized by reflections of similar magnitudes but different phases that approximately differ by π/2.
In another embodiment, the displacement d is chosen so that its image Md on the output curve is approximately equal to half the spacing Ω of the output images, and the grating is arranged to have a suitable periodic path length variation in adjacent arms so as to cause ±π/4 opposite phase shift in adjacent arms, wherein a particular input signal of a particular wavelength is split into two interleaved output image sets displaced by Ω/2. The optical apparatus thereby forms a reflective imaging arrangement with two input waveguides characterized by four reflection coefficients, each reflection coefficient produced in a particular input waveguide by a signal applied to one of the two input waveguides, such that the wavelength response produced by each of the four reflection coefficients is characterized by two sets of interleaved wavelength intervals U and V, with one set representing passbands of efficient reflection, and the other set representing stopbands of substantially lower reflection.
In yet another embodiment, instead of including the above path length variation, a 2×2 coupler is added to the optical apparatus thereby forming a reflective arrangement which is characterized by four reflection coefficients, each reflection coefficient produced in a particular input waveguide by a signal applied to one of the two input waveguides. The reflective arrangement, in response to a multiple wavelength input signal applied to the first input port, produces a reflected signal back to the first input port which has a wavelength response characterized by two sets of interleaved wavelengths U and V, with one set U representing passbands of efficient reflection back to the first input port, and the other set V representing stopbands of substantially lower reflection back to the first input port.
In the drawings,
In the following description, identical element designations in different figures represent identical elements. Additionally in the element designations, the first digit refers to the figure in which that element is first located (e.g., 102 is first located in FIG. 1).
In a conventional imaging arrangement or router, the input and output ports are connected to the input and output sections 120 and 140 along portions of two circles that are typically referred to as the input 121 and output 141 circles. Here I simply refer to them as the input and output curves. For simplicity,
The result is a router that produces a wavelength dependent output image of each input signal. The location of each output image is determined by its wavelength λ and therefore, signals of different wavelengths from a particular input port give rise to separate images that can be received by different output ports. Typically optical fibers are used for applying input signals to the input ports and for extracting output signals from the output ports. In practice, several output ports will be needed, if the router is to send signals to different destinations. Similarly, several input ports will be needed, in order to receive signals from different inputs. In wavelength division optical networks, the different wavelengths would represent different communication channels.
In
where R is the distance of the radiating apertures from the arms' focal point O and λ/a is the angular spacing corresponding to Ω. The spacing Ω is a function of the wavelength. However this dependence can be typically ignored, provided that λ is close to a particular design wavelength λ0, in which case Ω can be considered to be approximately equal to the value Ω0 at this particular wavelength λ0. In the following I ignore for simplicity the above wavelength dependence. On the other hand, the phase shifts produced by the various arms typically have strong wavelength dependence. By varying the wavelength, the locations of the output images Pi-1, Pi, Pi+1 will vary along the output curve 141. Of greatest importance in a conventional router is the central image of highest intensity. This is the image closest to the central point O corresponding to the focal point of the arms. This image is produced inside the central zone, which is an interval of width Ω centered at O. The remaining images (higher orders) are produced in adjacent zones, of the same width. They typically have appreciably smaller intensity in all cases except when they are close to the boundaries of the central zone.
In a conventional router, all the output ports or waveguides are located inside the central zone (which in
characterized by essentially unity amplitude inside the central zone and essentially zero amplitude outside. Each arm in this case radiates its entire power inside the central zone, and unwanted images outside this item zone are effectively eliminated. In practice, such a rectangular function is difficult to realize, and a simpler design is typically used, producing the power pattern
As shown in
However, the above variation is highly undesirable for our purpose here. I therefore next construct a reflective arrangement that substantially eliminates this problem, by equally reflecting all significant images. This will essentially eliminate the above variation and, as a result, rectangular passbands will be realized to a good approximation.
My interleaver, in its most general form, is essentially a periodic device with period equal to an integer fraction of the grating free-spectral range λf. Specifically, the purpose of the interleaver is to separate an input signal consisting of equally spaced wavelength channels with center wavelengths
λ1, λ2, λ3, λ4, λ5, λ6, . . . (where λ1<λ2<λ3 . . . ),
into two sets of signals, consisting of the odd and even channels, with center wavelengths
λ1, λ3, λ5, . . . and λ2, λ4, λ6, . . .
characterized by twice the wavelength spacing of the original input signal. To this purpose, each transmission coefficient of the interleaver must be essentially characterized by periodic behavior, with equally spaced passbands separated by stopbands, and with maximally flat behavior in each passband and stopband. As stated earlier, the period is in general an integer fraction of the free-spectral range, but it is convenient to first consider the special case where the integer is one, since the general case is entirely analogous. Then, the wavelength spacing, for each of the above two sets of wavelengths, is equal to the free-spectral range λf.
In accordance with the present invention, I have adapted the wavelength router described earlier to realize a low loss wavelength interleaver, or slicer, having rectangular wavelength transfer functions. With reference to
As shown, the output curve 141 of router 500 includes two interleaved sets A and B of equally spaced reflectors 501, 502. In this example the two input locations P,Q are separated by d≡Ω/2 where Ω is the separation of the various images Pi+1, Pi, Pi-1 and Qi, Qi-1. The two sets A, B are also separated by Ω/2. Notice each set is slightly displaced from the output curve 141, and their displacements are properly chosen so as to effectively produce a phase shift of π/2 between the reflected interleaved signals from Pi+1, Pi, Pi-1 and Qi, Qi-1, which in
With reference to
To more easily discuss another aspect of the present invention, there is shown in
With reference to
My interleaver or slicer, in its simplest form, can be simply used as 1×2 splitter device whose wavelength behavior is characterized by periodic transmission coefficients. In this case, the purpose of the interleaver is simply to separate an input signal consisting of equally spaced wavelength channels with center wavelengths
λ1, λ2, λ3, λ4, λ5, λ6, . . .
into two signals, consisting of the odd and even channels
λ1, λ3, λ5, . . . and λ2, λ4, λ6, . . .
having twice the wavelength spacing of the original input signal. To this purpose, each transmission coefficient of the interleaver must be essentially characterized by periodic behavior, with equally spaced passbands separated by stopbands, and with maximally flat behavior in each passband and stopband. Moreover, the two transmission coefficients of the interleaver must have complementary behaviors. That is, the stopbands of either coefficient must correspond to the passbands of the other. More generally, my interleaver can be realized as a 2×2 device (as shown in
I have designed my interleaver by combining a 2×2 coupler with a wavelength router adapted with suitable reflective terminations. As previously discussed,
In a conventional router of
The need for a circulator in
As discussed previously, the purpose of a slicer is to separate an input signal consisting of wavelength channels λ1, λ2, λ3, λ4, . . . into two signals, consisting of the even and odd channels λ1, λ3, . . . and λ2, λ4, . . . . In the arrangement of
where a is the period of the grating.
The basic properties of the arrangements considered here are best understood by first considering a special case, the design of a reflective filter realized as shown in FIG. 7. The filter simply consists of an imaging arrangement with a set of reflectors located along the output image curve. Initially let the segmented reflector be replaced by a single reflector covering the entire image curve. Then all images are reflected back towards the grating, and the grating then recombines the various images into a single reflected image produced at the input waveguide location P. Then, a reflection coefficient close to unity will be produced in the input waveguide, and the reflection will be approximately wavelength independent. The above result clearly applies in general to any input signal including several wavelengths, and a similar result is obtained in
Consider the arrangement of
Notice, in the particular example of
So far for simplicity I assumed that the various images are produced with unity magnification, in which case the separation d of the two input waveguides is equal to the corresponding separation of their output images. Moreover, the two separations were chosen equal to Ω/2, in which case the channel spacing for either set of output channels λ1, λ3, . . . and λ2, λ4, . . . becomes equal to the free-spectral range λf. More generally, the above channel separation can be chosen equal to an integer fraction of λf, in which case each pair of reflectors A, B must be spaced by an integer fraction of Ω/2, and d becomes an integer fraction of Ω/(2M), where M is the magnification. Also notice that I have neglected so far the small wavelength dependence of Ω. Thus in practice the above separations must be determined by using for Ω a particular value Ω0 corresponding to a particular design wavelength.
I have thus realized a slicer (also called interleaver) characterized by a set of equally spaced intervals such that the even numbered channels of the input signal I will be transferred as output O1 to port 1. The output O1 of even numbered channels are transferred to the same waveguide 509 carrying the input signal, and therefore a circulator 510 is needed to separate the reflected channels (output 01) at port 507 from the input channels I at port 508.
With reference to
The operation of the arrangement of
While the arrangement of
As discussed earlier the 2×2 coupler 501 connected to the two input waveguides of the router in
Notice the image curve is typically a circle of finite radius R. Clearly, however, the above results also apply to the special case where the image curve is a straight line, which can be viewed as circle of very large radius.
So far I have considered a planar imaging arrangement using a waveguide grating. More generally, an equivalent imaging arrangement can be realized using an Echelle grating, which is a well known type of grating characterized by a large path length difference between adjacent elements. The Echelle grating can be incorporated in a planar imaging arrangement, or in a conventional imaging arrangement (possibly including a lens or a curved reflector combined with an Echelle) realized in ordinary free-space.
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